tree of the month Belfast Botanic Gardens

Friends of Belfast Botanic Gardens

Tree of the month series - this series of short articles about trees in the Belfast Botanic Gardens is compiled by members of the Friends group and Gardens staff. The series will gradually build up to a comprehensive archive of information about the trees in Belfast Botanic gardens. Each article will illustrate the tree in Belfast together with information from a range of sources elsewhere.

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Tree of the Month, May 2009

Weeping willow - Salix x sepulcralis 'Chrysocoma'

by Jonathan Pilcher

Two types of weeping willows are commonly seen in Ireland. Salix caprea 'pendula', the weeping goat willow or 'pussy willow' is a unpreposessing tree of urban front gardens. Its branches are thick and stiff and the tree seldom attains an elegant shape. In contrast, the weeping Babylon willow is a superb elegant tree, although eventually becomming too large for a small garden. The species Salix babylonica is not reliably hardy, but a cross between it and Salix alba is much hardier. It is this cross that is widely grown in Ireland. It is now known as Salix x sepulcralis, although you will find many other names in books.

The weeping willows grown in the Belfast Botanic Gardens are the variety 'Chrysocoma' a form with bright yellow twigs in early spring. The golden weeping willow was introduced to the UK around 1908 perhaps from Germany. The Babylon willow has nothing to do with Babylon - it is a native of China.

The popular tree books and even more so the world wide web, are full of confusion concerning willows and this one in particular. Willows, like hazel and alders, have catkins. Catkins are a normal feature of trees that are wind pollinated yet willows are described as insect pollinated. This is based on the fact that the individual flowers in the catkins produce nectar - which would clearly be of no value to a wind-pollinated tree. The truth is probably more complex. A study of Japanese willows suggested that in fine weather wind pollination was more important for seed production whereas in a wet and cloudy spring high seed set depends more on insect pollination

Willows are normally dioecious - that is they have male and female catkins on different trees. The weeping willow is described as an infertile male clone. However, as you can easily see on the trees in the Botanic Gardens, the weeping willow produces both male and female catkins on the same tree. Whether or not the clone is really infertile, as are many interspecific hybrids, seems unclear. An Australian government web site describes the weeping willow as sometimes producing seed and therefore a potential threat to wild habitats. So it seems that under certain circumstances this hybrid may produce viable seeds

As the photographs below show the differences between the male and female catkins are quite distinctive.


Mature weeping willow in Belfast Botanic Gardens	branches of weeping willow with catkins

female catkin	male catkin - detail

female catkin - detail showing stigmas	single stigma (left) and single anther from weeping willow catkins